Uncured adhesive elastomeric compositions suitable for bonding diverse substrates such as other elastomers, metals, plastics, glass, fibers, paper and fabrics, have been disclosed by the inventors hereof in application Ser. Nos. 07/575,990, filed Aug. 31, 1990 and 07/571,459, filed Aug. 21, 1990, both abandoned. It is often desirable to apply such compositions to the substrates in a flowable form. For example, in industrial operations such as automobile manufacturing, shoe manufacturing or rotomolding of rubber parts it is desirable to use such compositions to replace welding in an assembly-line-type process. Compositions having reduced viscosity so as to be pumpable through supply lines are desirably used.
As is taught in the above U.S. patent applications, adhesive rubber compositions can be prepared by the addition of certain polymeric adducts to elastomeric compositions. This invention involves the addition of low molecular weight elastomers to such elastomeric/polymeric compositions to produce flowable compositions.
The flowability of an adhesive elastomeric composition can be increased by using elastomers of low molecular weight and viscosity in the formulation, so as to increase the flowability of the mixture, however, in general, the flowability of such compositions cannot be increased without sacrificing tensile and bond strength.
Bond strength, as measured by lap shear tests, is a direct indication of adhesive strength. Theoretically bond strength cannot be greater than two times tensile strength. The present compositions achieve adhesiveness of up to about 1.4 times tensile strength. However, as the present invention shows, adhesiveness is not directly proportional to tensile strength.
As is known to the art, the degree of crosslinking is also not a direct measure of the tensile strength or bond strength of a composition. Too high a degree of crosslinking can make a composition brittle and promote cracks.
While greater crosslinking may improve tensile strength up to an optimum point which varies from elastomer to elastomer, most elastomers are cured beyond this optimum point, and thus crosslink enhancing coagents would not enhance tensile strength, but rather would lower tensile strength. Such coagents are usually used to improve compression set, improve modulus and decrease elongation, or to enable the use of fillers in the composition. When elastomers having an average molecular weight low enough to flow freely at reasonably low temperatures (e.g., less than about 100.degree.), are used to formulate the adhesive compositions, the cured product is less tough than would be desirable, i.e., elongation is greater than desirable at a given tensile or bond strength.
When nonliquid elastomers are cured in the presence of crosslink-enhancing agents, percent elongation decreases and modulus at 100.degree. increases. For example, Colorado Chemical Specialties, Inc. Bulletin CCS-107, "High Vinyl 1-2 Liquid Polybutadiene Ricon EPDM/EPM Coagents" discloses that when a preferred crosslink enhancing agent of this invention, Ricon 153.TM. of Advanced Resins, Inc. (formerly Colorado Chemical Specialties, Inc., Grand Junction, Colo.) is added at 10 phr (parts per hundred) to EPDM rubber formulations, percent elongation decreases from 440 to 250 and 100% modulus increases from 210 to 380. However, elongation percent for cured flowable adhesive rubber compositions should be less than about 200%, and preferably less than about 180%.
It was surprising to find that flowable compositions could be made with acceptable tensile strengths and that the adhesiveness of such compositions could be improved by the addition of crosslink-enhancing coagents, while maintaining good tensile strength and improving elongation properties (toughness).
Crosslink-enhancing coagents known to the art include liquid polybutadienes having the properties of high 1,2 vinyl content such as Ricon 153.TM. and Ricon 154.TM., polybutadiene products of Advanced Resins, Inc., Grand Junction, Colo., di, tri and tetra functional acrylates and methacrylates, triallylcyanurate (TAC), triallylisocyanurate (TAIC), triallyltrimellitate (TATM), and N,N-metaphenylenedimaleimide (HVA-2). These coagents have been used as crosslink-enhancing coagents primarily with peroxide-cured elastomers. HVA-2 and the Advanced Resin products have also been used with sulfur-cured elastomers. These agents have been used to make the elastomers harder and more resistant to swell. However, such coagents have not been known to increase the adhesiveness of an elastomer.